As our front-engine water-cooled Porsches age, there are two items under the hood that need attention – and failure to pay attention could result in leaving you stranded on the road or worse, with an engine fire. These are two things that I check first on every car and fix if needed.
First on the list is the fuel lines under the hood. The rubber lines tend to get brittle and either crack or get pinholes where the fuel leaks onto the engine. When that happens, fuel can get onto the headers (hot) or into the spark plug holes (sparky), and neither of these options is desirable.
Rubber Fuel Lines
Understanding the flow of fuel to and from the fuel rail is important to understanding the problem and therefore understanding the fix. The fuel pump is in the rear of the car next to the fuel tank. Since we are talking about fuel injection, the pump makes as much as 80 psi through the fuel filter and up to the fuel rail. At the fuel rail, the gasoline goes through a fuel damper and into the fuel rail, where it smooths out the pump pulses and pressurizes the rail, making pressurized fuel available to the injectors. At the other end of the fuel rail is a fuel pressure regulator which bleeds off any excess pressure and sends that excess fuel back to the tank.
The fuel damper is the one connected to the rubber hose with the threaded fitting, and the fuel pressure regulator is the one with the hose clamp on the rubber hose. The threaded fitting is required since the fuel at that connector is under high pressure; the return to the tank is low pressure, so a hose clamp works well. The threaded fitting is where the problems start.
In time I have seen the high-pressure hose fail at the fitting – it gets weak, swells up and eventually starts leaking. So then someone simply cuts the line at the fitting, gets a barbed fitting from the hardware store, and uses a hose clamp to put it back together. Then in time that repair fails. Fuel leaks, then fire.
Replacing the high-pressure-side hose so that there is a factory connector is not cheap as the rubber hose and accompanying hard line are a dealer part that is expensive, depending on your particular model. But there are several companies out there who provide rubber hose replacements that require you to cut the hard line, then use high-quality compression fittings to install the new rubber line. Priced about $100, these kits replace both rubber lines and could save your car. The average mechanic can do it in about an hour. I use Rennbay in the Ocala area since Travis makes whatever I need and ships right away.
Another area of concern is the rubber “fuel jumper line” associated with the fuel damper. This line is also subject to cracking and leaking, and should be replaced if suspect. Cost is just under $100, and there is no aftermarket fix for this.
Our cars have two sensors that read the flywheel and tell the DME 1) that the engine is turning and 2) how fast the engine is turning. Reference sensor failure is a common fault as these cars age. When a 944 quits running or won’t start, the backyard mechanic immediately sees that there is no fuel and blames the fuel pump. A new fuel pump doesn’t fix it. (How many ads have you seen online that say, “Not running. Needs new fuel pump.”?? My bet – reference sensor issue. The DME first checks to see if the engine is turning before it turns on the ignition and the fuel pump. If there is no signal, there is no fuel or spark.
The sensors are actually pretty simple technology that do not normally go bad. However, the connectors at the top of the engine, in the rear of the head is where many have problems. There is a lot of heat in that area, and as these cars age, the plastic connectors get brittle, crack and break, causing the signal to fail and the DME to lose the information that the car is running. The DME turns off the fuel pump and the ignition, car quits running and it won’t restart.
if the damaged connector is on the sensor side, you can simply replace the sensor, plug it in and go. However, if the harness side of the connector is damaged, a new sensor may not help. I have reassembled broken connectors carefully and used black tape to keep them together, but this fix doesn’t last very long – long enough to get me home, but that’s about it.
Lindsey Racing makes a replacement harness for the reference sensors that works pretty well. You replace the wires that go from the sensors to the DME with this piece. A word of caution, though – installation can be a bit tricky. At one end you have two shiny-new connectors that work quite well. The other has to be fed through the firewall to the big white DME connector. You have to then take that connector apart, remove the pins for the old wiring and replace them with the pins at the end of the new harness. Having a pin removal tool makes the process a lot easier, but it still means getting under the dash and working upside down. The last 924S on which I completed this repair put me on my head under the driver’s side dash – I removed the driver seat to make it easier. It was worth it.
The tell-tale indication that a reference sensor is the culprit for your no-start situation is the infamous “tach bump.” As you crank the car over, watch the tach. The needle should move slightly as the car is turning over – because it is getting a signal from the reference sensor! If the needle doesn’t move, you are dealing with a reference sensor problem. Makes sense now, huh?
I will sheepishly admit that I have purchased cars for very low dollars that only needed a reference sensor replacement, coming out with a nice, running car for a couple hundred dollars and a couple hours of work. But the point here is that you need to check those rubber hoses under the hood to make sure they are not minutes away from failure. Also check for backyard hose-clamp fixes and replace them properly – and immediately. Check the reference sensor connectors and replace anything that is cracking or broken before it leaves you on the side of the road.
Another Note on Reference Sensors…
I had a 1988 924S M030 that was my daily driver/track car until is just stopped one day at the mailbox in front of my driveway. I put the car in the garage and worked on it every weekend for four months. Reference sensors. Twice. New reference sensor wiring. Fuel pump. Coil. Complete engine wiring harness. Ignition switch. And more. You see, it wouldn’t start, but once in a while it would tease me with a tach bump and even start and run for a half a minute or so, then nothing. I was ready to push it out into the yard and set it on fire.
The reference sensors are just magnetic pickups, reading a change in the magnetic field when a piece of metal passes close by. On the flywheel there are small set screws that stick out of the edge of the flywheel in line with one of the sensors. As the set screw passes the magnetic field of the sensor, the sensor electronically tells the DME, “NOW!” each time that screw goes by. The DME sees that the engine is turning. The other sensor reads the speed of the engine in the same manner by reading the passing of the teeth on the starter ring. Simple.
In my case, a simple failure caused months of heartache. It seems that one of the pressure plate bolts broke the head off of it, and the head of the Allen bolt bounced around inside the bell housing for a bit, then out and gone – but not before hitting the little set screw. Break it off? No. Instead it broke it in half, then bent it slightly – leaving just enough so that once in a while the DME got a reading, bounced the tach needle and started the car.
So I replaced the flywheel with another that was on the shelf and all was well. And it had a lot of new, but unnecessary parts, and taught me a great lesson. I knew that it was a reference sensor problem, but when my “standard fix” didn’t work, I decided to go fishing for solutions and spent a whole lot of money and time that wasn’t needed. It wasn’t the sensor – it was the thing that the sensor was looking for. Lesson Learned.
We had another reference sensor issue on a race car where one of the sensors was actually damaged. The part of the sensor inside the flywheel was dented and cut. We put in a new sensor and went back on the track the next day, when the clutch failed. The failing clutch cut the sensor. The lesson here? Think everything through. Later we had a 944 come into the shop with a similarly damaged sensor – and a failing clutch.